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United States Patent |
5,574,616
|
Becker
,   et al.
|
November 12, 1996
|
Degaussing technique
Abstract
To degauss cassettes of magnetic tape, a magnetic field is applied to the
magnetic material while the magnetic material moves with respect to the
field in a single air gap of a flux density of at least 1,000 gauss and at
an angle between 20 degrees and 70 degrees from the horizontal of the
magnetic tape for a time priod of at least one second and the field is
alternated at a frequency of at least ten hertz. The magnetic material may
be rotated while it is in the field.
Inventors:
|
Becker; Donald G. (Lincoln, NE);
Etherton; David J. (Lincoln, NE)
|
Assignee:
|
Garner Industries, Inc. (Lincoln, NE)
|
Appl. No.:
|
437168 |
Filed:
|
May 8, 1995 |
Current U.S. Class: |
361/149; 361/151; 361/267 |
Intern'l Class: |
H01F 013/00 |
Field of Search: |
361/149,151,267
|
References Cited
U.S. Patent Documents
4423460 | Dec., 1983 | Jackson et al. | 361/151.
|
4639821 | Jan., 1987 | Littwin et al. | 361/151.
|
4751608 | Jun., 1988 | Schultz | 361/151.
|
4897759 | Jan., 1990 | Becker | 361/151.
|
5198959 | Mar., 1993 | Scholtysik et al. | 361/149.
|
5204801 | Apr., 1993 | Becker et al. | 361/151.
|
5270899 | Dec., 1993 | Saito | 361/151.
|
5416664 | May., 1995 | Becker et al. | 361/149.
|
Primary Examiner: Fleming; Fritz M.
Attorney, Agent or Firm: Carney; Vincent L.
Parent Case Text
RELATED CASES
This application is a continuation-in-part of application Ser. No.
07/996,674 filed Dec. 24, 1992, now U.S. Pat. No. 5,416,664 which is a
continuation-in-part of U.S. application Ser. No. 07/870,476, filed Apr.
17, 1992, in the names of Donald Gene Becker and David Joseph Etherton,
now U.S. Pat. No. 5,204,801 issued Apr. 20, 1994, and assigned to the same
assignee as this application.
Claims
What is claimed is:
1. A method of degaussing magnetic material comprising the steps of:
applying a magnetic field to the magnetic material while the magnetic
material moves with respect to the field in a single air gap of a magnetic
path with a flux density of at least 1,000 gauss and at an angle between
20 degrees and 70 degrees; and
alternating the field at a frequency of at least ten hertz for a sufficient
time so that the flux density of at least 1,000 gauss passes through each
portion of the magnetic material having a signal recorded on it at the
angle of between 20 degrees and 70 degrees for at least one and one-half
cycles of the alternating field.
2. A method in accordance with claim 1 further including the step of
rotating at least one of the magnetic material and the field with respect
to each other while the magnetic material is in the field.
3. A method in accordance with claim 2 further including the step of moving
a plurality of cassettes of magnetic material in succession through the
field.
4. A method in accordance with claim 3 wherein the field is at an angle of
45 degrees to the magnetic material.
5. A method according to claim 1 futher including the steps of positioning
pole faces of a source of magnetic flux so that the flux path between them
is at the angle of between 20 degrees and 70 degrees with respect to the
magnetic material.
6. A method according to claim 5 in which the pole faces are stationary
with respect to each other and the magnetic medium is moved with respect
to the pole faces while maintaining an angle of the peak field between the
pole faces between 20 degrees and 70 degrees with respect to the magnetic
medium.
7. A method according to claim 1 in which the magnetic field and magnetic
medium are rotated with respect to each other while maintaining an angle
of the peak field density of between 20 degrees and 70 degrees to the
magnetic material.
8. A method according to claim 1 in which a signal is recorded on the
magnetic medium prior to degaussing, and the method of degaussing reduces
energy transfer between even and odd harmonics of the recorded signal
during degaussing whereby a fundamental and all harmonics are reduced on
the magnetic medium below a predetermined level.
9. Apparatus for degaussing magnetic material comprising:
means for applying a magnetic field to the magnetic material with a flux
density of at least 1,000 gauss and at an angle between 20 degrees and 70
degrees with respect to the magnetic medium; and
means for alternating the field at a frequency of at least ten hertz for a
sufficient time so that the flux density of at least 1,000 gauss passes
through each portion of magnetic material having a signal recorded on it
at the angle of between 20 degrees and 70 degrees for at least one and
one-half cycles of the alternating field;
said means for applying magnetic material including a magnetic circuit
having a single air gap wherein a high intensity field in the air gap is
applied to the magnetic material.
10. Apparatus in accordance with claim 9 further comprising means for
rotating the magnetic material while it is in the field.
11. Apparatus in accordance with claim 10 further comprising means for
moving a series of magnetic cassettes into the field while maintaining the
field at an effective angle.
12. Apparatus in accordance with claim 11 in which the means for moving
includes a conveyor assembly.
Description
BACKGROUND OF THE INVENTION
This invention relates to methods and apparatuses for erasing information
from a magnetic recording medium and to improved electromagnets for that
purpose.
In one class of methods and apparatuses for erasing information from
magnetic recording media, the recording medium, which may be a magnetic
tape wound about a reel, is subjected to a varying or alternating
electromagnetic field to randomize the magnetic particles on the magnetic
material.
In one prior art method and apparatus in this class, the magnetic tape is
moved into an electromagnetic field that is applied in each of a plurality
of different directions, one direction at a time, such as by first
applying a vertically oriented field followed by a longitudinally oriented
field. Techniques of this type are described in U.S. Pat. No. 4,730,230
and U.S. Pat. No. 4,751,608.
In another prior art technique, the magnetic tape is carried by a conveyor
over a rotating electromagnet that has pole faces parallel to each other
in the same plane underneath the conveyor belt. Thus, the electromagnet
rotates a time varying electromagnetic field with it to cause the time
varying electromagnetic field to pass through the magnetic material in the
tape at a plurality of different angles. This type of prior art device is
disclosed in U.S. Pat. No. 4,639,821.
Still another prior art apparatus and technique of this class includes a
conveyor that carries a magnetic tape into a rotating magnetic field. The
rotating magnetic field is substantially parallel with the conveyor and is
in the plane of the magnetic tape. It is created by electromagnetic poles
on both sides of the conveyor belt, energized in such a way that similar
polarities oppose each other on opposite sides of the tape with the phases
of the poles on each side of the conveyor changing in synchronism to cause
the field to rotate. Thus, north electromagnetic poles face each other on
opposite sides of the tape and south magnetic poles face each other on
opposite sides of the tape and the north and south poles alternate with
each other in the same plane on the same sides of the tape. The poles
rotate in synchronism.
The prior art degaussing techniques provide erasure that is satisfactory
for some purposes but do not erase to the extent desired for other
applications. In general, the systems which apply vertical and
longitudinal fields separately have the disadvantage of moving the energy
back and forth between even and odd harmonics of the recorded signal. This
reduces the effectiveness of the erasure. Rotational fields by themselves
do not improve the depth of erasure to the extent needed for some
applications when practiced as described in the aforementioned prior art
references.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the invention to provide a novel degaussing
apparatus and method.
It is a further object of the invention to provide a novel technique for
applying an alternating current, electromagnetic field to a magnetic
medium.
It is a still further object of the invention to provide a technique for
increasing the depth of erasure of information recorded on a magnetic
medium over other techniques for erasing recorded information from a
magnetic medium without increasing the magnetic flux density.
In accordance with the above and further objects of the invention, a
magnetic field vector is applied through a tape cassette at an angle of
between 20 to 70 degrees to the direction of the orientation of magnetic
domains on the medium, or of course, the supplement of the angles in this
range. For the common longitudinally recorded magnetic tape, the field
vector is between 20 to 70 degrees from the longitudinal axis of a strip
of the tape. For a cassette having this orientation of recorded
information, the field may be applied at an angle of between 20 to 70
degrees to the larger flat sides of the cassette. Generally, if this is
done, the field is rotated about an axis that is perpendicular to the flat
sides of the cassette so that all sections of the tape receive a vector
with a proper orientation at some time during the rotation of the field.
The alternating field is applied at a sufficient magnetic flux density to
change the orientation of individual magnetic domains or particles and
thus randomize the orientation.
The field is applied parallel to a plane that is: (1) at an acute or obtuse
angle to the longitudinal direction of the tape; and (2) at an obtuse or
acute angle to the side-to-side direction of the tape. Thus, the field
vector is not perpendicular to the direction of magnetization (which is
generally along the longitudinal axis of the tape) nor is the field vector
in the direction of magnetization. However, it has vertical and
longitudinal components that are simultaneously present at the same point
in the tape.
The angle is selected so that when resolved into components, the magnetic
flux density has a vertical component nearly equal to or less than that of
the longitudinal component. Keeping the ratio of vertical component to
longitudinal component (the tangent of the vector angle) between 2.7 and
0.36 prevents energy transfer between even and odd harmonics of the
recorded signal during degausing and results in even erasure of all
harmonics. Preferably, angle is selected so that the signal fundamental
and all harmonics are reduced below a desired degauss level. In the
preferred embodiment, the angle is approximately 45 degrees from the
horizontal and the reel is rotated as it is moved linearly through the
field.
It can be understood that the degausser of this invention has the advantage
of providing a greater amount of erasure with the same flux density than
other techniques which apply fields in different directions in different
stages or apply them directly or predominantly in the direction of
magnetization or normal to the direction of magnetization.
SUMMARY OF THE DRAWINGS
The above noted and other features of the invention will be better
understood from the following detailed description when considered with
reference to the accompanying drawings in which:
FIG. 1 is a fragmentary, side elevational view of a degaussing apparatus
and tape to be degaussed in accordance with an embodiment of the
invention;
FIG. 2 is another fragmentary, side elevational view of the degaussing
apparatus of FIG. 1 in another state of its progression during degaussing;
FIG. 3 is a top view of the embodiment of FIG. 1;
FIG. 4 is a fragmentary, side elevational view of another embodiment of the
invention;
FIG. 5 is a top view of the embodiment of FIG. 4;
FIG. 6 is a fragmentary, side elevational, schematic view of another
embodiment of the invention;
FIG. 7 is a plan view of the embodiment of FIG. 6 shown in schematic form;
FIG. 8 is a simplified, fragmentary perspective view of another embodiment
of the invention; and
FIG. 9 is a fragmentary side elevational view of still another embodiment
of the invention.
DETAILED DESCRIPTION
In FIGS. 1 and 2, there is shown a fragmentary, simplified sectional view
of a degausser 10 in the process of receiving and degaussing a magnetic
tape cassette 12 containing magnetic tape for degaussing. The degausser 10
includes a source of a magnetic field 16, a conveyor assembly 14 for
delivering the magnetic tape cassette 12 into the magnetic field 16, and a
system shown at 18A and 18B for causing the field 16 and/or the cassette
12 to rotate with respect to each other. FIG. 1 shows the tape 12 entering
the degaussing apparatus and FIG. 2 shows the tape 12 within the field.
The source 16 of the magnetic field is positioned with respect to the
conveyor so that when the cassette 12 is moved into the field, the source
16 creates a field at an angle to the side of the cassette 12 that is in
the range of 20 to 70 degrees, and in the preferred embodiment, 45 or
fewer degrees. This angle is referred to in this specification as the
"effective angle". FIG. 1 shows one effective angle from one side and FIG.
2 shows a different effective angle that is the supplement of the
effective angle shown in FIG. 1. It is desirable for the domains of the
tape to receive peak or substantially a peak field strength at both the
effective angle and its supplement.
In using the effective angle, a magnetic field vector is applied through
the tape at an angle of between 20 to 70 degrees from the longitudinal
side edges of the tape. The alternating field is applied at a sufficient
magnetic flux density to change the orientation of individual magnetic
domains or particles and thus randomizes the orientation.
The field is applied parallel to a plane that is: (1) at an acute or obtuse
angle to the longitudinal direction of the tape; and (2) at an obtuse or
acute angle to the side-to-side direction of the tape. Thus, the field
vector is not perpendicular to the direction of magnetization (which is
generally along the longitudinal axis of the tape) nor is the field vector
in the direction of magnetization. However, the field vector has
components that are simultaneously present in the perpendicular and
longitudinal directions.
The effective angle is selected so that when resolved into components, the
magnetic flux density has a vertical component nearly equal to or less
than that of the longitudinal component. More generally, the ratio of the
vertical component to the longitudinal component (the tangent of the
vector angle) is preferably between 2.7 and 0.36. This reduces energy
transfer between even and odd harmonics of the recorded signal during
degausing and results in even erasure of all harmonics. The angle is
selected so that during degaussing, the fundamental and all harmonics are
reduced below a desired level.
In the preferred embodiment, the angle is approximately 45 degrees from the
horizontal and the reel is rotated as it is moved linearly through the
field. At 45 degrees, the vertical and longitudinal field components are
in equal proportions at a given point in space. Unlike other degaussers in
which the magnetic field is applied at a number of angles but at different
points in space or time resulting in ineffective degaussing of all
harmonics without consideration of the best angle, the degausser 10
applies the magnetic vector at the effective angle preferably all of the
time, but at least a sufficient proportion of the time the degausser
operates on the magnetic material to be efficient such as for example at
least 30 percent of the time the tape is in the field.
Of particular significance is that the field be at the effective angle for
the domains to be erased for at least one and one-half cycles and that it
be at the effective angle during at least one peak of the alternating
current as the tape and field are separated such as when a cassette is
moved from a field by a conveyor. The reduction of the field strength from
the fringes of the field at the time the tape and field are being
separated is particularly effective in erasing the tape.
The conveyor assembly 14 in the embodiment of FIG. 1 includes a belt 20, a
first roller 22 and a second roller 24. Either or both of the rollers are
drive rollers with the belt 20 passing over them as an endless belt
capable of moving the cassette 12 or a series of such cassettes along the
top run of the belt 20. Of course, any other means for moving the
cassettes with respect to the field may be used.
The source of the magnetic field 16 includes first and second
electromagnetic assemblies 30 and 32, respectively. One of the assemblies
32 in the embodiment of FIG. 1 is located close to the top run of the belt
20 and beneath the belt 20 so that the cassette 12 passes over it and the
other electromagnetic assembly 30 is located above the top run of the belt
20 at an elevation high enough so that the cassette 12 may pass
therebetween.
The first electromagnetic assembly 30 includes a first coil 34, a first
core leg 36, a bridging portion 38, a second coil 40 and a second core leg
42. The first and second coils 34 and 40 are respectively wound around the
first and second downwardly extending vertical legs 36 and 42 to create a
field through them with the bridge 38 connecting the legs 36 and 42 to
provide a closed magnetic path, to hold them in position, and to mount
them to the means for rotating 18B in fixed relationship with respect to
each other. The first core leg 36 ends in a pole face 54 and the second
core leg 42 ends in a pole face 56. The legs 36 and 42 are of high
permeability metal to concentrate the field at the pole faces 54 and 56
for extension downwardly through a cassette 12 to the second
electromagnetic assembly 32.
The second electromagnetic assembly 32 is constructed in a manner similar
to the first assembly 34 and includes a first coil 44, a first upwardly
extending, core leg portion 46, a bridge 48, a second coil 50 and a second
downwardly extending, core leg portion 52. The first leg 46 ends in a pole
face 58 and the second leg 52 ends in a pole face 60. The core legs 36,
42, 46 and 52 are parallel to each other with the pole faces 58 and 60
being positioned near the bottom of the bottom rung of the belt 20 and the
pole faces 54 and 56 being located at a sufficient height so that the
cassette 12 passes under them when carried into position by the belt 20.
The pole faces of opposite polarity on opposite sides of the top run of the
belt 20 are off-set from each other so that a field extending between the
pole face 54 and 58 and the field extending between the pole face 56 and
60 are at an angle to the cassette 12. That angle is between 20 and 70
degrees so that if the components of the field are resolved into
components that are vertical to the cassette 12 and horizontal to it, the
vertical component would be 0.36 to 2.7 times the magnitude of the
longitudinal component. Thus, the critical angle is formed for the
magnetic vector.
The bridges 38 and 48 are preferably of high permeability material and the
windings wound in a direction so that the poles alternate north to south.
In this case, the poles 58 and 60 are of opposite polarity and the pole 58
has the opposite polarity from the pole 54 so that if 54 is north, 58 is
south. In this embodiment, the reluctance path between the poles 54 and 56
and the reluctance of the path between the poles 58 and 60 should be much
greater than the reluctance of the path between the poles 54 and 58 and
the poles 56 and 60. This may be accomplished by spacing the poles 54 and
56 and the poles 58 and 60 much further from each other than the poles 54
and 58 and the poles 56 and 60 are spaced from each other.
The rotating system portions 18A and 18B are identical to each other and
each includes a motor driven shaft which serves to rotate the
electromagnetic assemblies in synchronism to maintain the effective angle
between the poles 54 and 58 and 56 and 60. For this purpose, the shaft 19B
and 19A are connected off-center to the bridges 38 and 48, respectively,
to support the bridges and thus, the core legs and windings as they are
rotated.
The shafts 19A and 19B are parallel to each other and perpendicular to the
bridges 48 and 38 so that the pole faces 54, 56, 58 and 60 remain parallel
to the belt 20. Power in this embodiment is electrically connected to the
windings through slip rings 21B and 21A connected to a source of A.C.
power such as the source 23 shown connected to the slip rings 21B with the
shaft 19B adapted to slide with respect to the slip rings and the slip
rings being electrically connected to the windings.
In FIG. 3, there is shown a simplified top view of the embodiment of FIG. 1
showing the cassette 12 being carried by the belt 20 into a position where
it is between the top positioned coils 34 and 40 and the bottom coils 44
and 50. The windings and conveyor are conventional. The belt 20 and
electromagnets 30 and 32 may be fabricated in the manner described in
connection with U.S. Pat. No. 4,897,759.
Either the cassette 12 or the electromagnetic assemblies 30 and 32 may be
rotated since it is the motion between the two that is significant and
similarly the tape 12 may be moved between the assembly or the assembly
moved over the tape 12. Moreover, instead of physically rotating the
electromagnets 30 and 32, a rotating magnetic field may be created, such
as that disclosed in U.S. Pat. No. 4,423,460, provided the angular
direction of the field as it passes through the cassettes is maintained at
the effective angle.
In FIG. 4, there is shown a simplified elevational sectional view of
another embodiment 10A of degausser positioned to erase a cassette 12
having a means for moving the cassette 12 and degaussing field with
respect to each other for erasing information from the magnetic tape, a
means for rotating the tape with respect to the field, and a source of a
magnetic field adapted to be applied at an angle to the tape for efficient
demagnetization thereof. In this embodiment, the means for moving the
cassette and degaussing field with respect to each other is similar to a
file cabinet drawer 74 positioned to move with respect to a source of a
magnetic field. The drawer 74 in this embodiment includes a drawer door
82, a drawer frame 84 and a drawer roller assembly 86.
The drawer frame 84 supports a rotating means 80 and is connected to the
door 82 for moving on the drawer rail and roller assembly 86. The source
of the magnetic field 16 (not shown in FIG. 3) is mounted to be stationary
with respect to the frame 84 so that the means for rotating the cassette
12 is moved between the first and second electromagnetic assemblies 30A
and 32A for demagnetization.
A means 80 for rotating the cassette 12 includes a pan mounted for rotation
within the frame 84 by bearings 87 such as along the rim of a circular
opening 89 in the bottom of the frame 84. To rotate the cassette 12, a
drive motor 90 is connected by a belt 92 to the rim of the pan 96 for
rotating about the idler pulley 94 so as to turn the pan. The tape mounts
within the pan 96 and is rotated therewith within the field 16. The first
and second electromagnetic assemblies 30A and 32A are mounted to the side
of the cabinet by frame members 70 and 72 (not shown in FIG. 3).
In FIG. 5, there is shown a plan view of the degausser 10A showing the
manner in which cassette 12 is mounted for rotation with the pan 96. The
pan 96 as best shown in FIG. 4, is positioned to be rotated by the motor
90, idler pulley 94 and belt 92 to rotate the cassette 12. The
electromagnetic assemblies 30A and 32A are identical to those in the
embodiment of FIGS. 1 and 2 except that they are mounted to be stationary
rather than rotatable.
In each of these embodiments, the tapes may be rotated or not. Slightly
better demagnetization is obtained by rotating the tapes, probably because
the intensity of the field at the effective angle is evenly distributed by
the rotation over the tape around the reel during the rotation. Otherwise,
the orientation of the tape within the field could cause some portions to
receive less activation than others.
In FIG. 6, there is shown a simplified schematic side elevational view
illustrating a novel electromagnetic assembly 30B utilized to demagnetize
a cassette 12. In this embodiment, the electromagnetic assembly 30B
includes a ferromagnetic core 100 with windings 21A and 21B mounted near
the pole pieces to form a single air gap 110 between them. This
arrangement is used in a manner analogous to the arrangements of FIGS. 1-5
except that, in the arrangements of FIGS. 1-5, there are two air gaps and
the pole pieces rotate with respect to each other.
With two gaps, the total length of the air gaps in the electromagnetic
circuit is inevitably longer than in the case of a single air gap, thus
increasing the reluctance of the magnetic circuits in the embodiments of
FIGS. 1-5 over that of the embodiments of FIGS. 6-9. The single gap of the
embodiment of FIGS. 6 and 7 and the two embodiments of FIGS. 8 and 9,
permits a shorter air gap and lower reluctance path and thus a lower
reluctance circuit. This enables greater field intensity with the same
amount of ampere turns and ultimately permits a higher intensity field.
In the embodiment of FIG. 6 a casette and electromagnetic assembly are
rotated with respect to each other and moved linearly with respect to each
other so that the alternating field within the gap is applied to every
segment of the tape at an angle at between 20 and 70 degrees, with the
angle when the gap is on one side of the casette being the suppliment of
the angle when it is on the other side of the cassette. As in the other
embodiments, the entire field is applied to the tape, including the fringe
portions of the field.
In FIG. 7 there is shown a top view of an embodiment similar to that of
FIG. 5 but utilizing the electromagnetic magnetic assembly 30B. In the
embodiment of FIG. 7 the electromagnetic assembly itself may be moved
linearly while the cassette is rotated or the cassette may be moved
linearly on a conveyor and rotated within a pan on the conveyor. The
relative simplicity of the electromagnetic assembly 30B permits an
uncomplicated mechanism for moving the electromagnetic assembly in a
cabinet or the like while the tape is rotated on a pan stationary with
respect to the housing.
In FIG. 8, there is shown a simplified perspective view of an
electromagnetic assembly 30C having the windings 21A and 21B on the ends
of a core 102 to form a gap 112. In the embodiment of FIGS. 6 and 7 the
gap 110 is formed by tilting the toroidal electromagnet around its major
axis whereas in the embodiment of FIG. 8 the electrical magnetic circuit
path is offset to form an air gap so the poles are not directly aligned in
the plan of the elliptical core 102. However, the operation of the
electromagnetic assemblies of FIGS. 6, 7 and 8 are substantially the same.
Similarly, in FIG. 9, a side elevational view is shown of still another
embodiment of electromagnetic assembly 30D having the windings 21A and 21B
to form a flex path in a core 104 with a gap 114. However, the gap in this
embodiment is formed by offsetting the open ends of the core from each
other.
In operation, a cassette or other holder for magnetic tape, is placed
either on the conveyor belt 20 in the embodiments of FIGS. 1 and 2 or in
the pan 96 in the embodiments of FIGS. 3 and 4. The cassette, or plurality
of cassettes, are then moved linearly into an alternating magnetic field
which alternates at a frequency of approximately 60 hertz in a field of
magnitue proportional to the magnetic coercivity of the cassette tape of
other medium to be degaussed. Satisfactory results, however, have been
obtained for a reduction of 95 decibels below saturation of a 750 oersted
coercivity tape using a field vector of only 2700 gauss.
While a 60 hertz field is generally used, other frequencies may be used
within the range of 10 to 400 hertz. The magnetic flux density should be
at least two times the tape or medium coercivity. For todays media, the
minimum field should be about 1,000 gauss. Generally the higher the field,
the better the depth of eraser, but other factors such as inductive
heating effects and power loss that increase with higher flux limit the
field strength in practical designs.
The tape is rotated in the rotating pan or the magnet assemblies are
rotated. The rate of rotation of the tapes, when used, may be as low as 40
revolutions per minute or higher. The linear movement of the tape, such as
on a conveyor belt with a plurality of tapes being moved, may be at any
convenient speed such as 0.3 inches per second but should be in a range of
0.1 inches per second to two inches per second. The rates of rotation and
linear speed are interdependent and are selected prior to the degaussing
in conjunction with the depth of erasure needed, the intensity of field
that is to be applied and the coercivity of the magnetic medium.
From the above description, it can be understood that the degausser of this
invention has several advantages, such as: (1) it is relatively economical
and fast in operation because it applies a single field at an effective
angle; and (2) it provides a greater level of erasure for the same
magnetic flux density than other techniques.
Although a preferred embodiment of the invention has been described with
some particularity, many modifications and variations in the invention are
possible in the light of the above teachings. Therefore, it is to be
understood that, within the scope of the appended claims, the invention
may be practiced other than as specifically described.
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